The present invention relates to a matrix line printer having an oscillating frame disposed for movement in front of a platen roller, the frame supporting a plurality of print elements arranged along a line which runs parallel to a line to be printed. The print elements are normally at rest but are movable by means of an energized electromagnet; moreover, the medium to be printed upon is movable after one or several printing passes in a direction transversely to the direction of a line.
Line printers of the type to which the invention pertains have to be distinguished from serial printers as far as the generation of the printing matrix is concerned. Rather than having a plurality of styli arranged in a vertical column and moving the printhead with these styli across the printing medium, i.e., along a line to be printed, the line printers have a plurality of print elements (styli) arranged along the entire length of a line but in spaced-apart relationship, the spacing being "bridged" by oscillating the carrier for the styli as stated. The vertical dimension of any character results from transporting the printing medium in the normal direction of advance, but in small steps. The number of print elements depends, of course, to some extent on the width of the print medium to be covered and may typically involve 30 or more print elements. These print elements as well as their respective drives are arranged in the above-mentioned oscillating frame; and they are arranged, of course, strictly in one line; they have to be moved as a unit in order to cover the entire field and all possible print positions. In particular, printing of columns is not restricted to locations where print elements happen to be in the resting position; but columns are to be printed, more or less, anywhere on the print medium, which is the reason that the oscillatory frame movement has to cover, at least approximately, the distance between two print elements. For dynamic reasons, a significant overshoot is needed.
German printed patent application No. B2 22 24 716 proposes a bar-like carrier element on which electromagnetic coils, each with a core, are mounted. The coils serve as drives for relatively long print needles or wire-like styli, constituting dot-printing elements. The armature of each electromagnetic device is of the plunger variety and is relatively heavy which means that the entire frame arrangment has considerable mass which, in turn, means that driving and retarding forces have to be correspondingly high. In practice, therefore, one may operate only at relatively low oscillating frequencies which, in turn, means that the printer is relatively slow. The problem is compounded by the size of the relatively long print needles which rstricts the print frequency and rate of needle actuation, independently from the heavy weight of the carrier and frame arrangement.
In order to increase the printing frequency, matrix line printers have been suggested (for example, U.S. Pat. No. 3,941,051) in which biased springs "shoot" the print element with point printing tips toward the printing medium; normally, a permanent magnet biases the springs into a deflective position; but by means of an electromagnet, the biasing field is temporarily offset, permitting the spring to relax and propel the print element toward the printing medium. Therefore, the printing speed, as far as the immediate actuation is concerned, depends upon the stored mechanical energy, and requires a very careful choice in the selection of the spring, the magnet, and the other magnetical control elements. Not only has such an arrangement proved to be quite expensive; but it was found also that operating with the principle of stored mechanical energy and bias offsetting a certain interference by magnetic stray flux is to be observed, and these effects retard, to some extent, the printing speed.